GB2285314A

GB2285314A - Device for calibrating and evaluating signals from exhaust gas oxygen probes

Info

Publication number: GB2285314A
Application number: GB9425832A
Authority: GB
Inventors: Gerhard Hoetzel
Original assignee: Robert Bosch GmbH
Current assignee: Robert Bosch GmbH
Priority date: 1993-12-30
Filing date: 1994-12-21
Publication date: 1995-07-05
Anticipated expiration: 2014-12-21
Also published as: DE4344961B4; JP3639626B2; DE4344961A1; GB9425832D0; US5524472A; FR2714729B1; FR2714729A1; GB2285314B; JPH07209245A

Abstract

A heated amperometric oxygen probe comprises a heater 18 and a probe signal measuring device which applies a predetermined voltage to the probe and ascertains as an uncorrected measuring signal S_N, a value representing the current which is flowing through the probe and which is indicative of the oxygen concentration. The resistance of the probe at a given temperature increases with age so that the measured current signal may be lower than the correct value. To compensate for this effect, the evaluation device measures the internal resistance of the probe by applying an a.c. voltage to the probe from circuit 20, and controls the voltage Uh applied to the heater 18 of the probe in such a way that the internal resistance of the probe remains substantially constant. A calibration and compensation device 22 stores, in a predetermined calibration operating state of the engine, a value Uh-0 which is indicative of the heater voltage required when the probe is new, and, in the same calibration state, determines the heater value Uh-M which is currently required to achieve the said internal resistance. A correction value determination device determines, from the change between Uh-0 and Uh-M, a correction value KF for correcting the measuring signal, and correction device 23 corrects the uncorrected measuring signal S_N to a corrected measuring signal S_K. <IMAGE>

Description

2285314

-1DESCRIPTION EVALUATION DEVICES FOR SIGNALS OF OXYGEN PROBES

The invention relates to evaluation devices for signals of amperometric oxygen probes which are disposed in the flow of the exhaust gas from an internal combustion engine.

The function of an amperometric or limiting current oxygen probe, hereinafter referred to as "oxygen probe" for the sake of brevity, will be described in general, with reference to the accompanying Fig. 5. for the measurement of the oxygen content of lean mixtures. The pumping current lp is plotted against the pumping voltage Up at a predetermined operating temperature. This operating temperature may be, for example, 8500C with an internal ohmic resistance of the probe of 100 Q. As soon as the pumping voltage is applied to the probe, 02 molecules, located in the exhaust gas which have entered a diffusion chamber of the cell, are reduced to 02ions which are them pumped through the probe material by the electric field applied thereto, and out of the diffusion chamber. The current is limited only by the ohmic resistance in the case of low pumping voltages, and for this reason the ip-Up, characteristic is linear in the first instance. However, if only very few 02 molecules are located in -2the exhaust gas, saturation of the pumping current occurs even at relatively low pumping voltages. This case is indicated in Fig. 5 by the bottommost solid line. The higher the saturation level, the leaner the mixture.

During actual use of such a probe in the flow of the exhaust gas from an internal combustion engine, the pumping voltage is typically fixedly predetermined with so large a value that the pumping current lies in the range of saturation. This current is thereby directly indicative of the oxygen content of the exhaust gas which has entered the diffusion chamber of the probe. To complete the picture, it may be mentioned that methods of measurement also exist which vary the pumping voltage in dependence upon the oxygen content,, namely,, the smaller the quantity of oxygen, the lower is the pumping voltage used. It will be seen directly from Fig. 5 that, although lower pumping voltages may actually be used for smaller oxygen contents, the condition is always fulfilled that measurement is performed in the saturation range of the characteristic. It is immaterial to the invention whether the pumping voltage is maintained constant or is varied in dependence upon the oxygen content, and for this reason no details regarding this will be discussed.

The characteristics shown by solid lines in Fig. 5 may apply to a new probe i.e. a probe in the new state. However. the internal resistance of a probe increases with increasing age of the probe. To clarify the description, it is assumed in Fig. 5 that, after a long period of operation. the internal resistance is only half the initial internal resistance, when measured at the same temperature. As is shown by a broken line in Fig. 5, the pumping current is then very considerably limited by the ohmic resistance and, with the pumping voltage which is applied in practical use and which is characterised by a vertical line in Fig. Si it does not yet reach its saturation value in the case of a very lean mixture. Thus, a lower current is measured, and not the limiting current determined by the actual oxygen concentration. thus indicating that the oxygen concentration is being measured incorrectly. In order to prevent this, the probe is controlled to a constant internal resistance of the electrochemical cell. The rise of the described characteristic is thereby maintained, even with aging of the probe, although the probe temperature rises as the age of the probe increases.

In addition to the internal resistance, the magnitude of the output signal of a probe is still -4greatly dependent upon the temperature. since the diffusion conditions change to a considerable extent with the temperature. In order to exclude measurement errors conditioned by temperature, it is known (see, for example. US Patent 4.708.777) to keep the probe temperature constant by way of the measurement of the internal resistance of the heater of the probe, or to measure the temperature of the probe and to correct the measuring signal, indicating the oxygen concentration, of the probe with the aid of the measured temperature (see, for example. the document DE-C- 38 40 248).

What has been said hitherto also applies essentially to two-cell oxygen probes in which 02 ions are pumped out of, or into, a diffusion chamber by way of a pumping cell, in order to be able to measure a lean or rich mixture. Pumping is effected in such a way that a second cell, the sensor cell, always shows a predetermined constant voltage. Even a probe of this kind exhibits an increase in the internal resistance with aging, with the risk that the pumping voltage available is no longer sufficient to perform the entire 02 transport actually required. These difficulties can be avoided with regulation to a constant internal resistance, although the above. mentioned problem of incorrect measurement by increase in temperature also arises here.

Amperometric oxygen probes, to which the Application is directed, include single-cell and two cell oxygen probes having the functions described above.

The object of the invention is to provide an evaluation device for the signal of a heated amperometric oxygen probe which is disposed in the flow of the exhaust gas from an internal combustion engine and which may compensate for measurement errors of the probe signal due to aging, so that even an aged probe may continue to be used, without an increase in the emission of toxic gas.

In accordance with the present invention there is provided an evaluation device for the signal of a heated amperometric oxygen probe which is disposed in the flow of exhaust gas from an internal combustion engine and which comprises a heater and a probe signal measuring device which in use applies a predetermined voltage to the probe and ascertains as an uncorrected measuring signal (S_N), a value representing the current which is flowing through the probe and which is indicative of the oxygen concentration, the evaluation device comprising a resistance measuring device which in use ascertains the internal resistance of the probe by applying an a.c. voltage to the probe, -6a control device which in use triggers the heater device of the probe in such a way that the internal resistance of the probe remains substantially constant, a compensation device having a new state determination device which in use determines and stores a new state value (Uh-0) which is indicative of the heater voltage as is required in the new state of the probe. in order to achieve the said internal resistance in a predetermined calibration operating state of the engine, an instantaneous state value determination device which in use determines an instantaneous state value (Uh-M) which is indicative of the heater voltage as is required in the instantaneous state of the probe, in order to achieve the said internal resistance in the calibration operating state of the engine, and a correction value determination device which in use determines, from the change in the instantaneous state value relative to the new state value, a correction value (KF) for the correction of the measuring signal, and a correction device which in use corrects the uncorrected measuring signal to a corrected measuring signal (S-K) by mathematical combination with the correction value.

This has the advantage that fresh calibrating measurements can always be performed at a predetermined operating state of the internal -7combustion engine, in order to be able to compensate for age- conditioned changes in the probe signal with the aid of a correction value. In this connection, it is particularly important that regulation is always made to maintain a constant internal resistance of the probe (not of the heater), and the heater voltage, required for this at the said predetermined operating state, is used as an indication of the state of aging of the probe. This procedure results in an increased heating of the probe as its age increases, that is, in order to be able to maintain its internal resistance at a constant value, which, on the one hand, has the result that the measuring signal is not falsified by a change in the internal resistance, although, on the other hand, has the result that the measuring signal is subjected to a temperature-conditioned error. However, the temperature-conditioned error may be derived very accurately from the change in the heater voltage, so that this error may be compensated in a very simple manner, without a temperature sensor being required for this purpose.

By way of example only, specific embodiments of the invention will now be described, with reference to the accompanying drawings, in which:- Fig. 1 is a block circuit diagram of an evaluation device with aging compensation for -8signal of a heated, limiting current oxygen probe; Fig. 2 is a flow diagram for explaining the operation, performed by the evaluation device of Fig. 1, for ascertaining a value indicating the new state of a probe; F1g. 3 is a flow diagram for explaining an operation detecting the aging state of a probe; Fig. 4 is a flow diagram for explaining an operation for the correction of the measuring signal of a probe with the use of the values detected by the operating sequences of Figs. 2 and 3; and Fig. 5 is a known pumping current/pumping voltage graph for a limiting current oxygen probe.

Fig. 1 shows an evaluation device 10 for the signal of a heated, limiting current probe 11 disposed in the exhaust gas pipe 12 of an internal combustion engine 13. A butterfly valve 15, from which a load signal L is tapped by a load signal detection device 16, is located in the intake manifold 14 of the engine. A rotational speed sensor 17 on the engine supplies a rotational speed signal n.

The probe 11 illustrated only diagrammatically, has a heater device 18 and two measuring electrodes 19.1 and 19.2 to which the above-mentioned pumping voltage Up is applied and through which the pumping current ip flows. The uncorrected aging signal -9output by the probe 11 is designated S-N.

The evaluation circuit 10 in accordance with Fig. 1 has an internal resistance measuring device 20, a heater voltage control device 21, a compensation device 22 and a correction device 23 in the form of multiplier. The compensation device 22 is in turn constructed from a newstate value store 24, a subtracter 25, a characteristic 26 and a correction value store 27.

The internal resistance measuring device 20 measures the internal resistance Rs of the probe 11 by applying an a.c. voltage in the range of, for example, 1 to 5 kHz to the probe electrodes 19.1 and 19.2. It outputs the measured resistance value to the heater voltage control device 21 where this value is compared as an actual value with a desired value for the internal resistance of the probe of, for example, 100 9. The heater voltage control device outputs a heater voltage Uh such that the measured internal resistance of the probe is held at the desired value as accurately as possible.

Since the internal resistance Rs of the probe continuously increases with increasing age of the probe at a specific temperature of, for example, 8500C but, on the other hand, decreases with increasing temperature, the constant maintaining of the internal -10resistance of the probe with the aid of the heater voltage control device 21 means that the temperature of the probe continuously increases as the probe becomes older. However. the uncorrected probe signal S_N is increasingly falsified as the temperature of the probe increases. This falsification has to be compensated, for which purpose the heater voltage Uh is used, in the way it has to be applied in a specific operating state of the engine. so that the desired internal resistance of the probe is established. This compensation is performed by the compensation device 22 in the manner now described.

After a vehicle has been put into operation for the first time, an aging calibration is carried out as soon as a predetermined calibration operating state of the engine is established, such as idling or overrun operation. It has to be an operating state in which, with the probe heating switched off. the probe 11 assumes a temperature in a very narrow range of temperature, such as 700C with a difference of only a few degrees. Namely, the heater voltage Uh, as it has to be established in order to obtain a predetermined internal resistance of the probe, is then indicative of the state of aging of the probe. This voltage would be, for example, 9 V in the new state of the probe, and a temperature of 850C would be obtained in -11order to reach the predetermined internal resistance of, for example, 100 Q. This voltage Uh-O is stored in the store 24 as a new state value.

During the subsequent actual operation of the vehicle, the calibration operation is always effected again whenever the engine enters the calibration operating state. The older the probe becomes, the higher is the temperature required in order to achieve the predetermined internal resistance of the probe of 100 Q. Since the engine always supplies the same quantity of heat in the calibration operating state. the quantity of heat required to achieve the higher temperature has to be applied entirely by the heater device 18 by applying a high heater voltage Uh.

The heater voltage Uh-M measured during an actual calibration operation is a value which indicates the instantaneous state of the probe. The new state value Uh-0 stored in the store 24 is subtracted from this voltage. and. with the aid of this difference, a correction factor KF-M is read from the characteristic 26, the uncorrected probe signal S-N being multiplied by this correction factor in the multiplier 23 in order to obtain a corrected probe signal S-K. The instantaneous value KF-M of the correction factor Kf is always smaller than one, which is the correction factor KF-0 in the new state, since the probe voltage -12has to be increased with increased age of the probe in order to maintain the predetermined internal resistance, and since the value of the probe signal increases at a constant oxygen concentration as the probe voltage, and thus the probe temperature, increases. Therefore, the uncorrected probe signal has to be reduced by the correction factor.

The operating sequences described above will now be additionally explained with reference to Figs 2 to 4.

Referring to Fig. 2, after a vehicle has been put into operation for the first time, it is continuously checked in a step s2.1 whether the vehicle is being operated for the first time in the calibration operating state. As soon as this is the case, the new state value Uh-0 at which the predetermined internal resistance of the probe is established, is measured in a step s2.2. This value is stored in a step s2.3. The value One for the correction factor K-F has already been stored in the correction factor store 27.

During the subsequent operation of the motor vehicle, it is continuously checked in a step 23.1 whether the engine has entered the calibration operating state. This is determined by comparing the actual values of the load signal L and of the rotational speed n with the predetermined value -13ranges. If both the actual values lie in the respective associated value ranges, this indicates that the calibration operating state exists. As soon as this is the case. the heater voltage Uh-M is measured in a step s3.2. The heater voltage control device 21 at the same time continues to run unchanged as in non-calibration operation, that is. always so that the predetermined internal resistance of the probe is maintained. The new state value Uh_0 is subtracted from the instantaneous state value Uh-M in a step s3.3, and, in a step sM, the correction factor Kf associated with this difference is read from the characteristic 26 and stored in the store 27.

During operation of the engine in operating states other than the calibration operating state. the uncorrected probe signal SN at the predetermined internal resistance of the probe is detected in a step s4,1. The correction factor Kf is read from the correction factor store 27 in a step s4.2 and is multiplied by the uncorrected probe signal in the multiplier 23. The corrected probe signal S-K is thereby available.

The construction described above is particularly advantageous when a substantially linear relationship exists between the change in the heater voltage and the change in the probe signal at a constant oxygen -14concentration. On the other hand, in this connection, not linearly, it may be more advantageous, in a modified characteristic. not to plot correction factors against the differential instantaneous state value minus new state value, but against the percentage instantaneous state value/new state value. In this case, a divider 25 exists instead of the subtracter 25. It is also possible to use a characteristic at the input side of the new state value store 24 instead of at the output of a subtracter or divider. This characteristic then contains compensation factors not plotted against a voltage difference or a voltage quotient, but directly against the heater voltage. A compensation factor is read for the new state from this characteristic and is stored. A fresh compensation factor is read out during a subsequent calibration state and is divided by the new state value compensation factor in order thus to provide the correction factor KF. Irrespective of how the correction factor is actually determined, it is only important that it corrects that change in the uncorrected probe signal which is caused by an increase in temperature which is in turn conditioned by an increase in the heater voltage which is in turn necessary in order to compensate for the increase in the internal resistance of the probe conditioned by aging.

In the embodiment, a correction factor is determined, that is, a correction value which is multiplicatively combined with the uncorrected probe signal S-W. However, it is also possible to ascertain a correction value which is then added to the probe signal. The manner in which the correction value is determined in each case, and the manner in which it is mathematically combined with the uncorrected measuring signal, is greatly dependent in practice upon the characteristics of the probe and of the entire heating system. It is only essential to maintain the internal resistance of the probe permanently constant, and to compensate for temperature errors, produced by this measure, by detecting changes in the heater voltage which are required to maintain the internal resistance constant despite the aging of the probe.

The evaluation device in accordance with the invention makes it possible to use a limiting current oxygen probe longer than hitherto despite aging, without resulting in a increased emission of toxic gas. however, even in this case, a probe cannot be used for an unlimited length of time. In order to detect the instant at which the probe should be exchanged, it may be advantageous to compare the correction factor with a threshold value. As soon as -16the correction factor exceeds the threshold value. it is indicated that an exchange should be ------- == -

Claims

CLAIMS 1. An evaluation device for the signal of a heated amperometric

oxygen probe which is disposed in the flow of exhaust gas from an internal combustion engine and which comprises a heater and a probe signal measuring device which in use applies a predetermined voltage to the probe and ascertains as an uncorrected measuring signal (S_N). a value representing the current which is flowing through the probe and which is indicative of the oxygen concentration. the evaluation device comprising a resistance measuring device which in use ascertains the internal resistance of the probe by applying an a.c. voltage to the probe, a control device which in use triggers the heater device of the probe in such a way that the internal resistance of the probe remains substantially constant. a compensation device having a new state determination device which in use determines and stores a new state value (Uh-0) which is indicative of the heater voltage as is required in the new state of the probe, in order to achieve the said internal resistance in a predetermined calibration operating state of the engine, an instantaneous state value determination device which in use determines an instantaneous state value (Uh-M) which is indicative of the heater voltage as is required in the -18instantaneous state of the probe, in order to achieve the said internal resistance in the calibration operating state of the engine, and a correction value determination device which in use determines, from the change in the instantaneous state value relative to the new state value, a correction value (KF) for the correction of the measuring signal, and a correction device which in use corrects the uncorrected measuring signal to a corrected measuring signal (S-K) by mathematical combination with the correction value.
2. An evaluation device as claimed in claim 1, wherein the correction device in use multiplies the uncorrected measuring signal (S-N) by the correction value.
3. An evaluation device constructed and adapted to operate substantially as hereinbefore described with reference to, and as illustrated in, the accompanying drawings.